1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor (SAS), particularly to a FET type SAS advantageously used for automobile air bag systems, anti-lock brake systems (ABS), etc.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 4-25764 disclosed a FET type SAS as shown in FIG. 1, in which a FET type SAS 110 comprises a substrate 102 having source and drain regions 103a and 103b formed so as to extend from the surface to the inside of the substrate, a gate insulating film 104 formed on the substrate surface including the source and drain regions 103a and 103b, and a movable gate electrode 101 formed in one end of a cantilever anchored at the other end to the substrate. The gate electrode 101 is sustained above the gate insulating film 104 of the substrate 102 with an air gap 105 left between the bottom of the gate electrode 101 and the gate insulating film 104.
The SAS 110 detects an acceleration to which it is subjected, by detecting a change in the drain current due to a displacement of the movable gate electrode 101 caused by the acceleration, as shown by the double-headed arrow.
However, even when the SAS 110 is not under the influence of an acceleration, the gate voltage generates an electrostatic attractive force acting between the gate electrode 101 and the substrate 102, occasionally causing the former to be undesirably brought into contact with the latter.
To prevent such an undesired contact from occurring, the only possible method which could have been used was either to provide the cantilever 101 with an increased spring constant or to provide the air gap 105 with an increased height. Either method, however, is not recommended because the sensitivity of the SAS would be reduced.